1. Field of the Invention
The present invention relates to an optical transmission technique in an optical communication system and more particularly, to a wavelength division multiplexing optical transmission apparatus which modulates light outputs of a plurality of light emitting elements whose optical wavelengths are shifted mutually slightly with individual electric signals to obtain light signals, and which combines these light signals into a single light transmission signal to be transmitted onto a single optical fiber cable.
2. Description of the Related Art
Active researches have been conducted on increase in the transmission capacity of an optical fiber communication system. An optical transmitter in an optical communication transmission system generally inputs a digital electric signal to a light intensity modulator to modulate light intensity of a continuous wave (CW) light signal received from a laser light source and to transmit the modulated light signal onto a transmission line. In order to increase the transmission capacity, it is necessary to perform the light intensity modulation at high speed. However, when the above object is tried to be realized only by improvement in driving frequency bands of IC and optical device, its limit is expected. To avoid this limitation, various methods have been suggested. Among them, in particular, a wavelength division multiplexing (WDM) system using a plurality of light sources having respective different oscillation wavelengths is drawing attention as a promising means in future. One of such prior art wavelength division multiplexing systems will be explained below.
FIG. 1 shows an example of arrangement of a prior art wavelength division multiplexing system. In the arrangement, n light sources 11 to 11n have different oscillation wavelengths of .lambda..sub.1 to .lambda..sub.n, respectively. To realize stabilization in oscillation wavelengths of the light sources, temperature control of a laser chip and fine adjustment of laser resonators in their intervals are carried out. Light intensity modulators 21 to 2n are provided which correspond to light signals of light sources 11 to 11n, respectively, to obtain data-modulated output light signals. All of these output light signals are combined by optical coupler 3, amplified by optical amplifier 5, and then sent onto a transmission line.
In the wavelength division multiplexing system, the intervals of the oscillation wavelengths of the respective light sources becomes one of important factors. When the light signals received from the respective light sources are different in light powers, a light receiver cannot transmit the light signals correctly. To avoid this, it is necessary to set the light signals within a wavelength region where the amplification factor of the optical amplifiers of the light transmitters is substantially constant; therefore the region of the light signal wavelengths is limited. For the purpose of realizing a wavelength division multiplexing system of a large capacity, it is only required to transmit a multiplicity of light signals having different wavelengths included in the limited light signal wavelength region. However, when the wavelength intervals are too narrow, a nonlinear effect within an optical fiber involves noise generation and a transmission distance must be inevitably set to be short. Further, even when an accurate optical filter is used in the light receiver, it becomes difficult to identify the wavelengths of the light signals, thus disabling the data reception.
For the purpose of make equal the light receiving powers of the respective light signals in the light receiver, there is a pre-emphasis method for providing previously-different powers for the signal light sources to overcome the unequal gain among the optical amplifiers. In this case, it becomes possible to transmit more light signals by enlarging the light signal wavelength range, the wavelength intervals being kept constant. However, this also involves another problem that the unequal gain of the optical amplifier caused by fluctuations of the oscillation wavelength unfavorably affects its output power or that, when the light receiver employs a fixed-type optical filter, this causes deterioration of signal receiving characteristics.